Tristeza Disease Complex

Susceptible rootstock and scion combinations infected with the virus show symptoms similar to those caused by other diseases that injure the roots or girdle the crown. Trees infected with tristeza show light green foliage, poor growth flushes, and some leaf drop. The trees may produce a heavy crop of smaller fruit because the girdling at the bud union prevents starch transport to the roots. Feeder roots die from the tip towards the main root. Diseased young trees bloom early and abundantly and begin producing fruit 1 to 2 years before healthy trees.

Tristeza diseases, including quick decline, seedlings yellows, and stem pitting are different syndromes caused by different isolates of the tristeza virus. They differ in their virulence and their reaction to different scion cultivar and to the rootstock upon which the scion is growing.

Tristeza virus is spread through budding and grafting or by aphids feeding on citrus. The melon aphid, Aphis gossypii, is the vector for all tristeza isolates (types) found in California; it does not however, transmit all isolates equally well.

Tristeza is widespread throughout Southern California, but so far the concerted efforts of various groups have maintained tristeza at very low levels in the San Joaquin and desert valleys.

Management of the tristeza complex depends largely on preventive measures, such as using tolerant rootstocks and tristeza-free propagation material. However, because of the insect vector, disease spread cannot be prevented completely. Symptoms of tristeza become more apparent during the hot summer months when increased water needs cannot be met by the declining root system.

Observe quarantine restrictions to avoid spreading tristeza. No plants or plant parts should be shipped from infected Southern California districts to areas where tristeza is not present or is localized, such as the San Joaquin or Coachella valleys.

When grafting or topworking, use only certified, virus-free budwood. The Citrus Clonal Protection Program (CCPP) provides virus-free and true-to-type bud lines to nurseries and growers in California. Contact your county agricultural commissioner's office for listings of nurseries participating in the CCPP program. Virus-free and true-to-type budwood is also available from the University of California; contact your local farm advisor for more information.

In Southern California, where tristeza is widespread, you may want to remove infected trees only when they become unproductive.